1. Field of the Invention
The present invention relates to a distributed constant circuit, a high-frequency circuit and a bias applying circuit using the same, and an impedance adjusting method.
2. Description of the Background Art
In recent years, as mobile communication has been rapidly developed, electronic waves having a great many frequencies have been required for the communication, and the frequencies of the electronic waves used in the mobile communication are shifting to a microwave band. Therefore, an amplifier used for a portable machine is constituted by a monolithic microwave integrated circuit (MMIC) and a microwave integrated circuit (MIC) modularized.
As anamplifier for amplifying a signal having a desired frequency, a bias applying circuit for applying a predetermined DC bias to the gate and the drain of a field-effect transistor (FET) is used. The bias applying circuit is constituted by a distributed constant line (hereinafter referred to as a xcex/4 line) having a length which is one-fourth the wavelength of a fundamental wave, for example.
When one end of the xcex/4 line is short-circuited to a ground potential in an AC manner, the other end thereof enters an open state with respect to the frequency of the fundamental wave (hereinafter referred to as a fundamental frequency). The xcex/4 line is widely applied to various types of circuits such as a distributor, a synthesizer, a directional coupler, and a filter in addition to the bias applying circuit to the FET.
However, the lower the fundamental frequency is, the larger the length of the xcex/4 line is, thereby increasing the size of a chip or a module at frequencies which are not more than several gigahertz. Therefore, a method of miniaturizing the xcex/4 line has been examined.
FIG. 37 is a diagram showing a xcex/4 line, and FIG. 38 is a diagram showing a conventional distributed constant circuit equivalent to the xcex/4 line. In FIG. 37, Z0 is the characteristic impedance of a xcex/4 line 100, and L0 is the length of the xcex/4 line 100. In FIG. 38, Z1 is the characteristic impedance of a line 101, L1 is the length of the line 101, and C1 is the capacitance value (capacitance) of capacitors 102 and 103.
In the distributed constant circuit shown in FIG. 38, the line 101 is connected between a node NA and a node NB, the node NA is grounded through the capacitor 102, and the node NB is grounded through the capacitor 103.
If the characteristic impedance Z1, the length L1 and the capacitance value C1 satisfy relations expressed by the following equations (12) and (13), the distributed constant circuit shown in FIG. 38 is equivalent to the xcex/4 line 100 shown in FIG. 37 at a fundamental frequency (see an article entitled by Tetsuo Hirota, Akira Minakawa, Masahiro Muraguchi, xe2x80x9cReduced-Size Branch-Line and Rat-Race Hybrids for Uniplanar MMIC""sxe2x80x9d, IEEE MTT. Vol. 38, No. 3, March 1990):                               Z          1                =                              Z            0                                sin            ⁢                                          2                ⁢                π                            λ                        ⁢                          L              1                                                          (        12        )                                          C          1                =                              1                          ω              ⁢                              xe2x80x83                            ⁢                              Z                0                                              ⁢          cos          ⁢                                    2              ⁢              π                        λ                    ⁢                      L            1                                              (        13        )            
where xcex is the wavelength of a fundamental wave, and xcfx89 is the angular velocity of the fundamental wave. In the foregoing equations (12) and (13), the length L1 of the line 101 can be arbitrarily selected, so that the length L1 of the line 101 can be reduced.
FIG. 39 is a circuit diagram of a bias applying circuit using the distributed constant circuit shown in FIG. 38. A bias applying circuit 110 shown in FIG. 39 functions as a drain bias applying circuit for applying a drain bias Vdd to a FET 200.
In the bias applying circuit 110 shown in FIG. 39, a line 101 is connected between a node NA and a node NB, and the node NA is grounded through a capacitor 111. The drain bias Vdd is applied to the node NA. The node NB is grounded through a capacitor 103, and is connected to the drain of the FET 200.
Z1 is the characteristic impedance of the line 101, and L1 is the length of the line 101. C1 is the capacitance value of the capacitor 103, and Cg is the capacitance value of the capacitor 111. Zfr is an impedance in a case where an input side (a terminal A) is viewed from the node NB, and Zlo is an impedance in a case where an output side (a terminal B) is viewed from the node NB. The impedance Zfr and the impedance Zlo are taken as 50xcexa9.
The capacitor 111 has a sufficiently small impedance relative to the fundamental frequency. Therefore, the node NA is short-circuited to a ground potential in an AC manner. Consequently, the node NB enters an open state with respect to the fundamental frequency. That is, the bias applying circuit 110 shown in FIG. 39 functions as a xcex/4 line with respect to the fundamental frequency. In this case, the drain bias Vdd is applied to the node NA.
On the other hand, when the xcex/4 line 100 of FIG. 37 is used as a drain bias applying circuit to the FET, one end of the xcex/4 line 100 is grounded through a capacitor, and the other end is connected to the drain of the FET. In this case, the other end of the xcex/4 line 100 enters an open state with respect to the fundamental frequency, and enters a short-circuited state with respect to even-order harmonics.
It has been known that in load conditions under which a short-circuited state occurs with respect to even-order harmonics (particularly second harmonics) in a B-class operation, the power-added efficiency of an amplifier which is constituted by a FET is improved. When the A /4 line 100 is used as a bias applying circuit, therefore, the efficiency of the amplifier can be increased.
In a case of an A-class or AB-class operation of an amplifier, however, the conditions are not necessarily most suitable. In this case, it is necessary to adjust a harmonic impedance (particularly second harmonics) such that the characteristics of the amplifier are most suitable (see xe2x80x9cA Load Pull system with Harmonic Tuningxe2x80x9d, Microwave Journal, pp. 128-132, March 1996).
Meanwhile, when the distributed constant circuit shown in FIG. 38 is used as a bias applying circuit for a B-class amplifier as shown in FIG. 39, the node NB does not enter a short-circuited state with respect to even-order harmonics. Although an amplifier can be miniaturized, therefore, high efficiency of a B-class amplifier cannot be achieved.
Further, when the distributed constant circuit is used for an A-class or AB class operation, high efficiency can not be achieved because a harmonic impedance is fixed.
In the amplifier which is constituted by the FET, the FET may, in some cases, oscillate in a high-frequency region. As measures to prevent the FET from oscillating, there is a method of significantly decreasing gain at an oscillation frequency. When the xcex/4 line 100 shown in FIG. 37 is used as a bias applying circuit, the gain of the amplifier can be decreased at even-order harmonics, while the gain thereof at the other frequencies cannot be decreased. Therefore, a bias applying method capable of decreasing gain at an arbitrary frequency is demanded.
Furthermore, in an amplifier and a mixer, spurious (a signal having an unnecessary frequency) may, in some cases, be a problem. Therefore, measures to suppress spurious signals is demanded.
An object of the present invention is to provide a distributed constant circuit which has characteristics equivalent to a xcex/4 line with respect to a fundamental wave, can be miniaturized, and can suppress an arbitrary frequency, and a high-frequency circuit using the same.
Another object of the present invention is to provide a bias applying circuit which can be miniaturized and increased in efficiency.
Still another object of the present invention is to provide an impedance adjusting method for adjusting a load impedance of a transistor in a bias applying circuit.
A further object of the present invention is to provide a distributed constant circuit which can be miniaturized and lowered in cost.
A distributed constant circuit according to the present invention comprises a first line, a first capacitor, a second line connected in series with the first capacitor, a second capacitor, and a third line connected in series with the second capacitor, one end of the first line being connected to a predetermined reference potential through a series connection between the first capacitor and the second line, and the other end of the first line being connected to the reference potential through a series connection between the second capacitor and the third line, characteristics equivalent to a line having a length which is one-fourth a wavelength corresponding to a first frequency being obtained with respect to the first frequency, and the first capacitor and the second line resonating and the second capacitor and the third line resonating with respect to a second frequency different from the first frequency.
In the distributed constant circuit, the characteristics equivalent to the line having a length which is one-fourth the wavelength corresponding to the first frequency are obtained with respect to the first frequency. When one of the one end and the other end of the first line is short-circuited to the reference potential in an AC manner, therefore, the other of the one end and the other end of the first line enters an open state with respect to the first frequency.
The first capacitor and the second line resonate and the second capacitor and the third line resonate with respect to the second frequency. Therefore, the one end and the other end of the first line are short-circuited to the reference potential with respect to the second frequency.
In this case, the parameters of the first, second and third lines and the first and second capacitors are adjusted, thereby making it possible to shorten the first, second and third lines as well as to arbitrarily set the second frequency.
Consequently, there is provided a distributed constant circuit which has characteristics equivalent to a xcex/4 line, can be miniaturized, and can suppress an arbitrary frequency.
It is preferable that the characteristic impedance Za of the first line, the length La of the first line, the characteristic impedance Zb of the second and third lines, the length Lb of the second and third lines, the capacitance value C of the first and second capacitors, a first frequency f1, a wavelength xcex1 corresponding to the first frequency, a second frequency f2, and a wavelength xcex2 corresponding to the second frequency satisfy relations expressed by equations (1), (2) and (3):                               L          a                =                                            λ              1                                      2              ⁢              π                                ⁢          arctan          ⁢                      {                                          1                                  Z                  a                                            ⁢                              (                                                      1                                          2                      ⁢                      π                      ⁢                                              xe2x80x83                                            ⁢                                              f                        1                                            ⁢                      C                                                        -                                                            Z                      b                                        ⁢                    tan                    ⁢                                                                  2                        ⁢                        π                                                                    λ                        1                                                              ⁢                                          L                      b                                                                      )                                      }                                              (        1        )                                          L          b                =                                            λ              2                                      2              ⁢              π                                ⁢          arctan          ⁢                      1                                          C                ·                2                            ⁢              π              ⁢                              xe2x80x83                            ⁢                              f                2                            ⁢                              Z                b                                                                        (        2        )                                          Z          a                =                              Z            0                                sin            ⁢                                          2                ⁢                π                                            λ                1                                      ⁢                          L              a                                                          (        3        )            
In the distributed constant circuit, by satisfying the equation (3), voltage/current characteristics equivalent to the line having a length which is one-fourth the wavelength corresponding to the first frequency are obtained with respect to the first frequency.
By satisfying the equation (2), the first capacitor and the second line resonate and the second capacitor and the third line resonate with respect to the second frequency. Therefore, the one end and the other end of the first line are short-circuited to the reference potential with respect to the second frequency.
Furthermore, by satisfying the equation (1), when one of the one end and the other end of the first line is short-circuited to the reference potential in an AC manner, the other of the one end and the other end of the first line enters an open state with respect to the first frequency.
In this case, the parameters of the first, second and third lines and the first and second capacitors are adjusted, thereby making it possible to shorten the first, second and third lines as well as to arbitrarily set the second frequency.
Consequently, there is provided a distributed constant circuit which has characteristics equivalent to a xcex/4 line, can be miniaturized, and can suppress an arbitrary frequency.
A distributed constant circuit according to another aspect of the present invention comprises a first line, a capacitor, and a second line connected in series with the capacitor, one end of the first line being connected to a predetermined reference potential in an AC manner, and the other end of the first line being connected to the reference potential through a series connection between the capacitor and the second line, characteristics equivalent to a line having a length which is one-fourth a wavelength corresponding to a first frequency being obtained with respect to the first frequency, and the capacitor and the second line resonating with respect to a second frequency different from the first frequency.
In the distributed constant circuit, the characteristics equivalent to the line having a length which is one-fourth the wavelength corresponding to the first frequency are obtained with respect to the first frequency. Consequently, the other end of the first line enters an open state with respect to the first frequency.
The capacitor and the second line resonate with respect to the second frequency. Therefore, the other end of the first line is short-circuited to the reference potential with respect to the second frequency.
In this case, the parameters of the first and second lines and the capacitor are adjusted, thereby making it possible to shorten the first and second lines as well as to arbitrarily set the second frequency.
Consequently, there is provided a distributed constant circuit which has characteristics equivalent to a xcex/4 line, can be miniaturized, and can suppress an arbitrary frequency.
It is preferable that the characteristic impedance Za of the first line, the length La of the first line, the characteristic impedance Zb of the second line, the length Lb of the second line, the capacitance value C of the capacitor, a first frequency f1, a wavelength xcex1 corresponding to the-first frequency, a second frequency f2, and a wavelength xcex2 corresponding to the second frequency satisfy relations expressed by equations (1), (2) and (3):                               L          a                =                                            λ              1                                      2              ⁢              π                                ⁢          arctan          ⁢                      {                                          1                                  Z                  a                                            ⁢                              (                                                      1                                          2                      ⁢                      π                      ⁢                                              xe2x80x83                                            ⁢                                              f                        1                                            ⁢                      C                                                        -                                                            Z                      b                                        ⁢                    tan                    ⁢                                                                  2                        ⁢                        π                                                                    λ                        1                                                              ⁢                                          L                      b                                                                      )                                      }                                              (        1        )                                          L          b                =                                            λ              2                                      2              ⁢              π                                ⁢          arctan          ⁢                      1                                          C                ·                2                            ⁢              π              ⁢                              xe2x80x83                            ⁢                              f                2                            ⁢                              Z                b                                                                        (        2        )                                          Z          a                =                              Z            0                                sin            ⁢                                          2                ⁢                π                                            λ                1                                      ⁢                          L              a                                                          (        3        )            
In the distributed constant circuit, by satisfying the equation (3), voltage/current characteristics equivalent to the line having a length which is one-fourth the wavelength corresponding to the first frequency are obtained with respect to the first frequency.
By satisfying the equation (2), the capacitor and the second line resonate with respect to the second frequency. Therefore, the other end of the first line is short-circuited to the reference potential with respect to the second frequency.
Furthermore, by satisfying the equation (1), the other end of the first line enters an open state with respect to the first frequency.
In this case, the parameters of the first and second lines and the capacitor are adjusted, thereby making it possible to shorten the first and second lines as well as to arbitrarily set the second frequency.
Consequently, there is provided a distributed constant circuit which has characteristics equivalent to a xcex/4 line, can be miniaturized, and can suppress an arbitrary frequency.
The one end of the first line may be connected to a bias voltage, and the other end of the first line may be connected to an electrode of a transistor.
The first frequency may be the frequency of a fundamental wave, and the second frequency may be higher than the frequency of second harmonics relative to the fundamental wave.
A distributed constant circuit according to still another aspect of the present invention comprises a first line, a first capacitor, a second line connected in series with the first capacitor, a first impedance element, a second capacitor, a third line connected in series with the second capacitor, and a second impedance element, one end of the first line being connected to a predetermined reference, potential through a series connection between the first capacitor and the second line and connected to the reference potential through the first impedance element, and the other end of the first line being connected to the reference potential through a series connection between the second capacitor and the third line and connected to the reference potential through the second impedance element, characteristics equivalent to a line having a length which is one-fourth a wavelength corresponding to a first frequency being obtained with respect to the first frequency, and the first capacitor and the second line resonating and the second capacitor and the third line resonating with respect to a second frequency different from the first frequency.
In the distributed constant circuit, the characteristics equivalent to the line having a length which is one-fourth the wavelength corresponding to the first frequency are obtained with respect to the first frequency. When one of the one end and the other end of the first line is short-circuited to the reference potential in an AC manner, therefore, the other of the one end and the other end of the first line enters an open state with respect to the first frequency.
The first capacitor and the second line resonate and the second capacitor and the third line resonate with respect to the second frequency. Therefore, the one end and the other end of the first line are short-circuited to the reference potential with respect to the second frequency.
In this case, the parameters of the first, second and third lines and the first and second capacitors are adjusted, thereby making it possible to shorten the first, second and third lines as well as to arbitrarily set the second frequency.
Consequently, there is provided a distributed constant circuit which has characteristics equivalent to a xcex/4 line, can be miniaturized, and can suppress an arbitrary frequency.
It is preferable that the characteristic impedance Za of the first line, the length La of the first line, the characteristic impedance Zb of the second and third lines, the length Lb of the second and third lines, the capacitance value C of the first and second capacitors, the impedance Zc of the first and second impedance elements, a first frequency f1, a wavelength xcex1 corresponding to the first frequency, a second frequency f2, and a wavelength xcex2 corresponding to the second frequency satisfy relations expressed by equations (4), (5) and (6):                               L          a                =                                            λ              1                                      2              ⁢              π                                ⁢          arctan          ⁢                      {                                                                                j                    ⁢                                          xe2x80x83                                        ⁢                                          Z                      c                                                                            Z                    a                                                  ⁢                                  (                                                            1                                              j                        ⁢                                                  xe2x80x83                                                ⁢                        2                        ⁢                        π                        ⁢                                                  xe2x80x83                                                ⁢                                                  f                          1                                                ⁢                        C                                                              +                                          j                      ⁢                                              xe2x80x83                                            ⁢                                              Z                        b                                            ⁢                      tan                      ⁢                                                                        2                          ⁢                          π                                                                          λ                          1                                                                    ⁢                                              L                        b                                                                              )                                                                              Z                  c                                +                                  1                                      j2π                    ⁢                                          xe2x80x83                                        ⁢                                          f                      1                                        ⁢                    C                                                  +                                  j                  ⁢                                      xe2x80x83                                    ⁢                                      Z                    b                                    ⁢                  tan                  ⁢                                                            2                      ⁢                      π                                                              λ                      1                                                        ⁢                                      L                    b                                                                        }                                              (        4        )                                          L          b                =                                            λ              2                                      2              ⁢              π                                ⁢          arctan          ⁢                      1                                          C                ·                2                            ⁢              π              ⁢                              xe2x80x83                            ⁢                              f                2                            ⁢                              Z                b                                                                        (        5        )                                          Z          a                =                              Z            0                                sin            ⁢                                          2                ⁢                π                                            λ                1                                      ⁢                          L              a                                                          (        6        )            
In the distributed constant circuit, by satisfying the equation (6), voltage/current characteristics equivalent to the line having a length which is one-fourth the wavelength corresponding to the first frequency are obtained with respect to the first frequency.
By satisfying the equation (5), the first capacitor and the second line resonate and the second capacitor and the third line resonate with respect to the second frequency. Therefore, the one end and the other end of the first line are short-circuited to the reference potential with respect to the second frequency.
Furthermore, by satisfying the equation (4), when one of the one end and the other end of the first line is short-circuited to the reference potential in an AC manner, the other of the one end and the other end of the first line enters an open state with respect to the first frequency.
In this case, the parameters of the first, second and third lines and the first and second capacitors are adjusted, thereby making it possible to shorten the first, second and third lines as well as to arbitrarily set the second frequency.
Consequently, there is provided a distributed constant circuit which has characteristics equivalent to a xcex/4 line, can be miniaturized, and can suppress an arbitrary frequency.
Each of the first and second impedance elements may comprise an impedance device.
In this case, the parameters of the first and second impedance elements are adjusted in addition to the parameters of the first, second and third lines and the first and second capacitors, thereby making it possible to shorten the first, second and third lines as well as to arbitrarily set the second frequency.
Consequently, the distributed constant circuit can have characteristics equivalent to the xcex/4 line, can be miniaturized, and can suppress an arbitrary frequency.
The first and second impedance elements may be shifts of the impedances from a 50 ohm system in a case where circuits connected to one end and the other end of the first line are respectively viewed from the one end and the other end.
In this case, even when the impedances in a case where the circuits connected to the one end and the other end of the first line are respectively viewed from the one end and the other end are shifted from the 50 ohm system, the distributed constant circuit can have characteristics equivalent to the xcex/4 line, can be miniaturized, and can suppress an arbitrary frequency.
A distributed constant circuit according to a further aspect of the present invention comprises a first line, a capacitor, a second line connected in series with the capacitor, and an impedance element, one end of the first line being connected to a predetermined reference potential in an AC manner, and the other end of the first line being connected to the reference potential through a series connection between the capacitor and the second line and connected to the reference potential through the impedance element, characteristics equivalent to a line having a length which is one-fourth a wavelength corresponding to a first frequency being obtained with respect to the first frequency, and the capacitor and the second line resonating with respect to a second frequency different from the first frequency.
In the distributed constant circuit, the characteristics equivalent to the line having a length which is one-fourth the wavelength corresponding to the first frequency are obtained with respect to the first frequency. Consequently, the other end of the first line enters an open state with respect to the first frequency.
The capacitor and the second line resonate with respect to the second frequency. Therefore, the other end of the first line is short-circuited to the reference potential with respect to the second frequency.
In this case, the parameters of the first and second lines and the capacitor are adjusted, thereby making it possible to shorten the first and second lines as well as to arbitrarily set the second frequency.
Consequently, there is provided a distributed constant circuit which has characteristics equivalent to a xcex/4 line, can be miniaturized, and can suppress an arbitrary frequency.
It is preferable that the characteristic impedance Za of the first line, the length La of the first line, the characteristic impedance Zb of the second line, the length Lb of the second line, the capacitance value C of the capacitor, the impedance Zc of the impedance element, a first frequency f1, a wavelength xcex1 corresponding to the first frequency, a second frequency f2, and a wavelength xcex2 corresponding to the second frequency satisfy relations expressed by equations (4), (5) and (6):                               L          a                =                                            λ              1                                      2              ⁢              π                                ⁢          arctan          ⁢                      {                                                                                j                    ⁢                                          xe2x80x83                                        ⁢                                          Z                      c                                                                            Z                    a                                                  ⁢                                  (                                                            1                                              j                        ⁢                                                  xe2x80x83                                                ⁢                        2                        ⁢                        π                        ⁢                                                  xe2x80x83                                                ⁢                                                  f                          1                                                ⁢                        C                                                              +                                          j                      ⁢                                              xe2x80x83                                            ⁢                                              Z                        b                                            ⁢                      tan                      ⁢                                                                        2                          ⁢                          π                                                                          λ                          1                                                                    ⁢                                              L                        b                                                                              )                                                                              Z                  c                                +                                  1                                      j2π                    ⁢                                          xe2x80x83                                        ⁢                                          f                      1                                        ⁢                    C                                                  +                                  j                  ⁢                                      xe2x80x83                                    ⁢                                      Z                    b                                    ⁢                  tan                  ⁢                                                            2                      ⁢                      π                                                              λ                      1                                                        ⁢                                      L                    b                                                                        }                                              (        4        )                                          L          b                =                                            λ              2                                      2              ⁢              π                                ⁢          arctan          ⁢                      1                                          C                ·                2                            ⁢              π              ⁢                              xe2x80x83                            ⁢                              f                2                            ⁢                              Z                b                                                                        (        5        )                                          Z          a                =                              Z            0                                sin            ⁢                                          2                ⁢                π                                            λ                1                                      ⁢                          L              a                                                          (        6        )            
In the distributed constant circuit, by satisfying the equation (6), voltage/current characteristics equivalent to the line having a length which is one-fourth the wavelength corresponding to the first frequency are obtained with respect to the first frequency.
By satisfying the equation (5), the capacitor and the second line resonate with respect to the second frequency. Therefore, the other end of the first line is short-circuited to the reference potential with respect to the second frequency.
Furthermore, by satisfying the equation (4), the other end of the first line enters an open state with respect to the first frequency.
In this case, the parameters of the first and second lines and the capacitor are adjusted, thereby making it possible to shorten the first and second lines as well as to arbitrarily set the second frequency.
Consequently, there is provided a distributed constant circuit which has characteristics equivalent to a xcex/4 line, can be miniaturized, and can suppress an arbitrary frequency.
The impedance element may comprise an impedance device.
In this case, the parameter of the impedance element is adjusted in addition to the parameters of the first and second lines and the capacitor, thereby making it possible to shorten the first and second lines as well as to arbitrarily set the second frequency.
Consequently, the distributed constant circuit can have characteristics equivalent to the xcex/4 line, can be miniaturized, and can suppress an arbitrary frequency.
The impedance element may be a shift of the impedance from a 50 ohm system in a case where a circuit connected to the other end of the first line is viewed from the other end.
In this case, even when the impedance in a case where the circuit connected to the other end of the first line is viewed from the other end is shifted from the 50 ohm system, the distributed constant circuit can have characteristics equivalent to the xcex/4 line, can be miniaturized, and can suppress an arbitrary frequency.
The one end of the first line may be connected to a bias voltage, and the other end of the first line may be connected to an electrode of a transistor.
The first frequency may be the frequency of a fundamental wave, and the second frequency may be higher than the frequency of second harmonics relative to the fundamental wave.
A high-frequency circuit according to another aspect of the present invention comprises a transistor, a bias applying circuit for applying a DC bias to one electrode of the transistor, and a matching circuit for performing impedance matching between the electrode of the transistor and the other circuit, the bias applying circuit being constituted by any one of the above-mentioned distributed constant circuits, the matching circuit being provided between the bias applying circuit and the other circuit.
In the high-frequency circuit, the bias applying circuit is constituted by any one of the distributed constant circuits, thereby making it possible to transmit a signal having a first frequency between the electrode of the transistor and the other circuit while suppressing a second frequency and to apply a DC bias to the electrode of the transistor.
In this case, the matching circuit is provided between the bias applying circuit and the other circuit, so that frequency characteristics of a reflection coefficient at a node between the matching circuit and the other circuit have a wide peak directed downward at the first frequency. Consequently, wide band characteristics centered around the first frequency are obtained.
A high-frequency circuit according to still another aspect of the present invention comprises a transistor, a bias applying circuit for applying a DC bias to one electrode of the transistor, and a matching circuit for performing impedance matching between the electrode of the transistor and the other circuit, the bias applying circuit being constituted by any one of the above-mentioned distributed constant circuits, the matching circuit being provided between the electrode of the transistor and the bias applying circuit.
In the high-frequency circuit, the bias applying circuit is constituted by any one of the distributed constant circuits, thereby making it possible to transmit a signal having a first frequency between the electrode of the transistor and the other circuit while suppressing a second frequency and to apply a DC bias to the electrode of the transistor.
In this case, the matching circuit is provided between the electrode of the transistor and the bias applying circuit, so that frequency characteristics of a reflection coefficient at a node between the bias applying circuit and the other circuit have a narrow peak directed downward at the first frequency. Consequently, narrow band characteristics centered around the first frequency are obtained.
The high-frequency circuit may further comprise a harmonic removing circuit connected to the electrode of the transistor for removing a harmonic component relative to the first frequency.
In this case, it is possible to reliably remove the harmonic component relative to the first frequency while transmitting the first frequency between the electrode of the transistor and the other circuit.
A bias applying circuit according to another aspect of the present invention for bringing one electrode of a transistor into an open state with respect to the frequency of a fundamental wave and applying a DC bias to the electrode of the transistor comprises a resonance circuit connected between the electrode of the transistor and a predetermined reference potential, the resonance frequency of the resonance circuit being higher than the frequency of the second harmonics relative to the fundamental wave.
In the bias applying circuit, a DC bias is applied to the one electrode of the transistor, and the electrode of the transistor enters an open state with respect to the frequency of the fundamental wave. Further, the resonance circuit is connected between the electrode of the transistor and the reference potential, so that the electrode of the transistor enters a short-circuited state with respect to the resonance frequency of the resonance circuit. Consequently, the component of the resonance frequency of the resonance circuit is suppressed in the electrode of the transistor. Particularly, the resonance frequency of the resonance circuit is set to a frequency higher than the frequency of the second harmonics relative to the fundamental wave, so that losses are reduced in an AB-class operation of the transistor, thereby achieving high efficiency.
A bias applying circuit according to still another aspect of the present invention for applying a DC bias to one electrode of a transistor comprises any one of the above-mentioned distributed constant circuits, a first frequency being the frequency of a fundamental wave, a second frequency being higher than the frequency of second harmonics relative to the fundamental wave.
The bias applying circuit comprises any one of the distributed constant circuits, so that it is possible to transmit a signal having the first frequency between the electrode of the transistor and the other circuit while suppressing the component of the second frequency and to apply the DC bias to the electrode of the transistor.
In this case, the first frequency is the frequency of the fundamental wave, and the second frequency is set to a frequency higher than the frequency of the second harmonics relative to the fundamental wave, so that losses are reduced in an AB-class operation of the transistor, thereby achieving high efficiency. Consequently, there is provided a bias applying circuit which can be miniaturized and increased in efficiency.
An impedance adjusting method according to another aspect of the present invention comprises the step of changing the impedance of a resonance circuit in the above-mentioned bias applying circuit, to adjust a load impedance in second harmonics.
In the impedance adjusting method, it is possible to adjust the load impedance in the second harmonics by changing the impedance of the resonance circuit in the bias applying circuit. Consequently, it is possible to control the efficiency of a transistor.
An impedance adjusting method according to still another aspect of the present invention comprises the step of adjusting a load impedance in second harmonics on the basis of the product of a current and a voltage in an electrode in the above-mentioned bias applying circuit.
In the impedance adjusting method, it is possible to adjust the load impedance in the second harmonics on the basis of the product of a current and a voltage in the electrode in the bias applying circuit. Consequently, it is possible to control the efficiency of a transistor.
A distributed constant circuit according to another aspect of the present invention comprises a line and a capacitor, one end of the line being connected to a predetermined reference potential in an AC manner, and the other end of the line being connected to the reference potential through the capacitor, the line and the capacitor constituting an inductor with respect to a predetermined frequency.
In the distributed constant circuit, the capacitor and the short line constitute an inductor. Consequently, it is possible to miniaturize the circuit and lower the cost thereof.
It is preferable that the characteristic impedance Za of the line, the length La of the line, the capacitance value C of the capacitor, a wavelength xcex1 corresponding to the predetermined frequency, and an angular frequency xcfx891 corresponding to the predetermined frequency satisfy a relation expressed by an equation (7):                     1         greater than                               ω            1                    ⁢                      CZ            a                    ⁢                      tan            ⁡                          (                                                                    2                    ⁢                    π                                                        λ                    1                                                  ⁢                                  L                  a                                            )                                                          (        7        )            
The distributed constant circuit functions as an inductor by satisfying the equation (7).
A distributed constant circuit according to still another aspect of the present invention comprises a line, a capacitor, and an inductor component connected in series with the capacitor, one end of the line being connected to a predetermined reference potential in an AC manner, and the other end of the line being connected to the reference potential through a series connection between the capacitor and the inductor component, the line, the capacitor and the inductor component constituting an inductor with respect to a first frequency.
In the distributed constant circuit, the capacitor, the inductor component and the short line constitute an inductor. Consequently, it is possible to miniaturize the circuit and lower the cost thereof.
It is preferable that the characteristic impedance Za of the line, the length La of the line, the capacitance value C of the capacitor, the inductance L of the inductor component, a wavelength xcex1 corresponding to the first frequency, and an angular frequency xcfx891 corresponding to the first frequency satisfy a relation expressed by an equation (8):                               1                                    ω              1                        ⁢            C                           greater than                                             ω              1                        ⁢            L                    +                                    Z              a                        ⁢                          tan              ⁡                              (                                                                            2                      ⁢                      π                                                              λ                      1                                                        ⁢                                      L                    a                                                  )                                                                        (        8        )            
The distributed constant circuit functions as an inductor by satisfying the equation (8).
It is preferable that the capacitance value C of the capacitor, the inductance L of the inductor component, and an angular frequency xcfx892 corresponding to a second frequency satisfy a relation expressed by an equation (9):                                           ω            2                    ⁢          L                =                  1                                    ω              2                        ⁢            C                                              (        9        )            
In this case, by satisfying the equation (9), the other end of the line is short-circuited to the reference potential with respect to the second frequency. Therefore, it is possible to suppress the second frequency.
A distributed constant circuit according to a further aspect of the present invention comprises a first line, a capacitor, and a second line connected in series with the capacitor, one end of the line being connected to a predetermined reference potential in an AC manner, and the other end of the line being connected to the reference potential through a series connection between the capacitor and the second line, the first line, the capacitor and the second line constituting an inductor with respect to a first frequency.
In the distributed constant circuit, the capacitor and the short first and second lines constitute an inductor. Consequently, it is possible to miniaturize the circuit and lower the cost thereof.
It is preferable that the characteristic impedance Za of the first line, the length La of the first line, the characteristic impedance Zb of the second line, the length Lb of the second line, the capacitance value C of the capacitor, a wavelength xcex1 corresponding to the first frequency, and an angular frequency xcfx891 corresponding to the first frequency satisfy a relation expressed by an equation (10):                               1                                    ω              1                        ⁢            C                           greater than                                             Z              b                        ⁢                          tan              ⁡                              (                                                                            2                      ⁢                      π                                                              λ                      1                                                        ⁢                                      L                    b                                                  )                                              +                                    Z              a                        ⁢                          tan              ⁡                              (                                                                            2                      ⁢                      π                                                              λ                      1                                                        ⁢                                      L                    a                                                  )                                                                        (        10        )            
The distributed constant circuit functions as an inductor by satisfying the equation (10).
It is preferable that the characteristic impedance Zb of the second line, the length Lb of the second line, the capacitance value C of the capacitor, a wavelength xcex2 corresponding to a second frequency, and the angular frequency xcfx892 corresponding to the second frequency satisfy a relation expressed by an equation (11):                               1                                    ω              2                        ⁢            C                          =                              Z            b                    ⁢                      tan            ⁡                          (                                                                    2                    ⁢                    π                                                        λ                    2                                                  ⁢                                  L                  b                                            )                                                          (        11        )            
In this case, by satisfying the equation (11) , the other end of the first line is short-circuited to the reference potential with respect to the second frequency. Therefore, it is possible to suppress the second frequency.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.